1. Field of the Invention
This invention relates to an oxide-coated cathode for CRT and a manufacturing method thereof, and more particularly to an oxide-coated cathode and its manufacturing method which can enhance the electron emissive characteristics and the longevity of the cathode by improving the distribution features of the electron-emissive substance with an ion implantation method.
2. Description of the Prior Arts
In conventional structure of a cathode for cathode ray tube as shown in FIG. 1, a cap 1, applied with an electron-emissive substance, e.g., a carbonate 2, is secured to a sleeve 3 by welding. The electron emissive substance is prepared as following.
A ternary carbonate of BaCO.sub.3, CaCO.sub.3 and SrCO.sub.3 made from an alkaline earth metal elements of Ba, Sr and Ca is mixed with a binder and a solvent to make a suspension. This suspension is sprayed onto a base made of Ni as a major element containing small amounts of a reducing element such as Mg or Si, The applied suspension is heated in a vacuum to convert it into the ternary composite oxide which is used as an electron-emissive substance
That is, such conversion that the ternary carbonate is heated in a vacuum can be expressed by the following reaction formula. ##STR1## The CO.sub.2 gas generated from the above reaction is discharged by a evacuation pump, and the ternary carbonate is converted into the ternary composite oxide of(Ba,Ca,Sr)O.
When said the ternary composite Oxide is aged at a temperature of 700-800.degree. C., it is reduced at the interface with the base of Ni by the reducing element to produce free Ba. This free Ba may play a role as a donor which contributes to the electron emission operation.
Here, when the operation of the electron emission may be continued for a long period, the ternary composite oxide of (Ba,Ca,Sr)O reacts with the reducing element in the base of Ni to produce an oxide layer. Such oxide layer is so called "intermediate layer" which is a composite oxide layer composed of MgSiO.sub.3 or BaSiO.sub.3, etc. Said intermediate layer comes to inhibit the diffusion of the reducing element, whereby free Ba generation is limited. Accordingly, such a conventional cathode has defects of poor electron emissive characteristics and short longevity.
To solve the above mentioned defects, various prior arts are proposed in U.S. Pat. No. 4,797,593 or 4,864,187, Japanese Patent Laid Open SHOWA 61-271,732, 63-254,635, 64-77,819, or HEISEI 1-102,829. These patents disclose at least one of the technics that one or two elements in the group of rare earth metal elements such as In, Ga, or Sc, and the like is added to the ternary carbonate by using a dispersion method, a deposition method, or coprecipitation method in order to increase the generation of free Ba, thereby lengthening the longevity of the cathode and providing with high current density.
For example, Scandium(Sc) which is added to the ternary carbonate acts in reducing the intermediate layer of composite oxide which raises the above mentioned defects, thereby it provides with an effect of limiting the formation of the intermediate layer.
However, as described in Japanese Patent Laid open SHOWA 64-77,819 and HEISEI 1-102,829, the dispersion method where Sc is mixed with the ternary carbonate in a powder state of Sc.sub.2 O.sub.3, or the deposition method where the ternary carbonate is deposited in the Scandium solution has the below defect; the distribution characteristics of the Sc is poor due to the differences of specific gravity and cohesiveness between the respective elements.
Moreover, the coprecipitation method where Sc and the ternary carbonate are concurrently deposited and precipitated therefrom results in similar problems as in the conventional cathode owing to the creterion of the amount of Sc for deposition.
Meantime, in order to solve the aforementioned defects, there is proposed with a technique where Sc is coated on the surface of the base of Ni to form a layer by using a sputtering method or a heat evaporation method. But such a method has a problem that the coated Sc layer on the base acts as an obstruction layer to isolate the carbonate from the reducing element so that the generation of the free Ba is inhibited. Another art where small amounts of Sc is directly added during the manufacturing procedure of the base of Ni is proposed, which results in raising the manufacturing costs.